Long-Lived Mars Rover Opportunity Keeps Finding Surprises

The ground texture seen in recent images from the rover resembles a smudged
version of very distinctive stone stripes on some mountain slopes on Earth that
result from repeated cycles of freezing and thawing of wet soil. But it might
also be due to wind, downhill transport, other processes or a combination.

Opportunity landed on Mars in January 2004. As it reaches the 5,000th Martian day, or sol, of what was planned as a 90-sol mission, it is investigating a channel called "Perseverance Valley," which descends the inboard slope of the western rim of Endeavour Crater.

"Perseverance Valley is a special place, like having a new mission
again after all these years," said Opportunity Deputy Principal
Investigator Ray Arvidson of Washington University in St. Louis. "We
already knew it was unlike any place any Mars rover has seen before, even if we
don't yet know how it formed, and now we're seeing surfaces that look like
stone stripes. It's mysterious. It's exciting. I think the set of observations
we'll get will enable us to understand it."

On some slopes within the valley, the soil and gravel particles appear
to have become organized into narrow rows or corrugations, parallel to the slope,
alternating between rows with more gravel and rows with less.

The origin of the whole valley is uncertain. Rover-team scientists are
analyzing various clues that suggest actions of water, wind or ice. They are
also considering a range of possible explanations for the stripes, and remain
uncertain about whether this texture results from processes of relatively
modern Mars or a much older Mars.

Other lines of evidence have convinced Mars experts that, on a scale of
hundreds of thousands of years, Mars goes through cycles when the tilt or
obliquity of its axis increases so much that some of the water now frozen at
the poles vaporizes into the atmosphere and then becomes snow or frost
accumulating nearer the equator.

"One possible explanation of these stripes is that they are relics
from a time of greater obliquity when snow packs on the rim seasonally melted
enough to moisten the soil, and then freeze-thaw cycles organized the small
rocks into stripes," Arvidson said. "Gravitational downhill movement
may be diffusing them so they don't look as crisp as when they were fresh."

Bernard Hallet of the University of Washington, Seattle, agrees the
alignments seen in images of Perseverance Valley are not as distinctive as the
stone stripes he has studied on Earth. Field measurements on Earth, near the
summit of Hawaii's Mauna Kea where the soil freezes every night but is often
dry, have documented how those form when temperature and ground conditions are
right: Soils with a mix of silt, sand and gravel expand more where the
finer-grain material is most prevalent and retains more water. Freezing expands
the soil, pushing larger particles up. If they move to the side, as well as
down the general slope, due to gravity or wind, they tend to move away from the
finer-grain concentrations and stretch out downslope. Where larger particles
become more concentrated, the ground expands less. The process repeats hundreds
or thousands of times, and the pattern self-organizes into alternating stripes.

Perseverance Valley holds rocks carved by sand blowing uphill from the
crater floor, and wind might also be the key in sorting larger particles into
rows parallel to the slope.

"Debris from relatively fresh impact craters is scattered over the
surface of the area, complicating assessment of effects of wind," said Opportunity
science-team member Robert Sullivan of Cornell University, Ithaca, New York.
"I don't know what these stripes are, and I don't think anyone else knows
for sure what they are, so we're entertaining multiple hypotheses and gathering
more data to figure it out."

Every sol Opportunity keeps working may add information to help solve some
puzzles and find new ones. For more information
about Opportunity, visit: